1. Field of the Invention
The present invention relates to a shock absorber mounted between a bumper and a frame of a vehicle to transform impact energy applied to the bumper into deformation energy to thereby absorb the energy.
2. Description of the Prior Art
Conventional shock absorbers include a cylinder type as described in U.S. Pat. No. 4,537,734 and a type using plastic deformation such as buckling. In the cylinder type, impact energy is absorbed as energy that compresses a cylinder. The latter type is described in Japanese Patent Laid-Open No. 086309/1997, and in which impact energy is transformed into deformation energy of a member, whereby the energy is absorbed. The cylinder type has the advantage that the energy absorbing characteristics are stable. However, this design needs precision and many components. Therefore, it has the disadvantage that the member is heavy and expensive. While, the type utilizing plastic deformation has the advantage that it is light and inexpensive, but this type is inferior to the cylinder type in energy absorbing property.
The shock absorber utilizing plastic deformation is lightweight and cheap, and so it meets the recent demand for reductions in vehicle fabrication costs. Accordingly, it is an object of the present invention to provide a shock absorber using plastic deformation, which is stable in energy absorbing property and is light in weight and is mechanically simple and inexpensive to manufacture. As a result, we have developed a shock absorber that is mounted between a bumper and a frame of a vehicle and acts to transform impact energy applied to the bumper into deformation energy, thereby absorbing the energy. This shock absorber is characterized in that it is a multi-diameter stepped tube consists of different diameter tube portions formed by partially reducing or partially enlarging a straight tube that can be plastically deformed. Each different diameter tube portions joined by steps formed between edge of each different diameter tube portions. One end of the multi-diameter stepped tube being connected to said bumper, and the other end of the multi-diameter stepped tube being connected to the frame of the vehicle, respectively. With this shock absorber, the bumper is normally held to the vehicle frame as a member that supports the bumper.
The shock absorber of the present invention is designed to absorb impact energy with the process that the small outside diameter tube portion is pushed into the large outside diameter tube portion. A part of the impact energy compresses the tube portions. However, the impact energy is mostly spent as plastic deformation of the step that is dragged in as the small outside diameter tube portion is pushed into the large outside diameter tube portion. In this way, the energy is absorbed.
The multi-diameter stepped tube is comprised as follows, namely,
(a) the step formed between the edge of the tube portions are folded back to each tube portions, and
(b) tube portions are joined by the step that the inside diameter of one tube portion is greater than the outside diameter of the other.
Concreteley, the shock absorber of the present invention comprises as follows,
In the (a), the step has already started to be plastically deformed. Therefore, the energy necessary for initial plastic deformation can be made small. Hence, the inward movement of the step that is dragged in progresses smoothly.
In the (b), the adjacent tubes easily telescope. Consequently, plastic deformation of the step progresses well. That the inside diameter of small outside diameter tube portion is greater than the outside diameter of the other one means that the width W of the step is greater than the wall thickness t of the larger outside diameter tube portion. Preferably, the sizes of the tube portions that are connected by the step roughly satisfy the following relations:
t1 greater than t2 and W greater than t2
where t1 is the wall thickness of the small outside diameter tube portion, t2 is the wall thickness of the large outside diameter tube portion, and W is the width of the step that connects both tube portions. It is assumed that the small and large outside diameter tube portions have lengths of H1 and H2, respectively.
Concretely, the shock absorber of the present invention comprises as follows, (1) the shock absorber, wherein the diameter stepped tube is a two-diameter stepped tube, the two-diameter stepped tube forming a large outside diameter tube portion and a small outside diameter tube portion which are substantially circle obtained by partially reducing or partially enlarging a straight tube that can be plastically deformed, each edge of different diameter tube portions joined by steps in order that an axis of each different diameter tubes located on a substantially aligned, and the large outside diameter tube portion provided and fixed on the impact receiving part of the frame of the vehicle, and (2) the shock absorber, wherein the multi-diameter stepped tube is a three-diameter stepped tube, the multi-diameter stepped tube forming a small outside diameter tube portion, a medium outside diameter tube portion and a large outside diameter tube portion which are substantially circle obtained by partially reducing or partially enlarging a straight tube that can be plastically deformed, each edge of different diameter tube portions connected by steps in order that an axis of each different diameter tube portions located on a substantially aligned, the different diameter tube portions arranged in order of size of diameter, and the large outside diameter tube portion provided and fixed on the impact receiving part of the frame of the vehicle.
Although the shock absorber in accordance with the present invention is preferably of multi-diameter stepped structure, substantial restrictions are placed on the number of stages due to the installation space. The three-diameter stepped tube is a realistic structure in terms of the number of machining steps. For example, a shock absorber consisting of a three-diameter stepped tube can be easily obtained as follows. On a normal round metal pipe (a circular straight tube), a given length from one end of the pipe is enlarged in diameter, and other end is reduced in diameter. In this three-diameter stepped tube, the medium outside diameter tube portion suppresses tilt of the small outside diameter tube portion. The small and medium diameter tube portions, respectively, can be together pushed into the large diameter tube portion.
It is assumed in the shock absorber comprising the three-diameter stepped tube that the small outside diameter tube portion has a length of H1 and a wall thickness of t1, the medium outside diameter tube portion has a length of H2 and a wall thickness of t2, the large outside diameter tube portion has a length of H3 and a wall thickness of t3, the step connecting the small and medium outside diameter tube portions has a width of W1, and the step connecting the medium and large outside diameter tube portions has a width of W2. Relations t1 greater than t2 greater than t3, W1 greater than t2, and W2 greater than t3 hold. As mentioned previously, where a normal round metal pipe is partially enlarged and reduced in diameter to form a three-diameter stepped tube, the wall thickness t1 of the small outside diameter tube portion obtained by reducing the diameter is inevitably large than the wall thickness t2 of the medium outside diameter tube portion. Also, the wall thickness t3 of the large outside diameter tube portion obtained by enlarging the diameter is unavoidably small than the wall thickness t2 of the medium outside diameter tube portion. One metal pipe is subjected to two plastic processing steps, i.e., increase of the diameter and decrease of the diameter. As a result, a shock absorber consisting of a desirable three-diameter stepped tube can be fabricated.